Process for making nickel-based superalloys

ABSTRACT

A process for making nickel-based superalloys that possess excellent high-temperature properties which includes the steps of providing a metal powder having a controlled amount of oxygen and carbon which is confined and densified at an elevated temperature forming a billet that can be further deformed, if desired, to provide an appropriate shaped component. Thereafter, the billet or deformed part is subjected to heat treatment to effect a growth in the grain size thereof to attain optimum physical properties, whereafter the alloy is carburized to increase the carbon content thereof to a level in excess of about 500 parts per million (ppm) which is performed in a manner so as to preferentially promote carbide formation at the grain boundaries of the alloy, thereby stabilizing the alloy against further grain growth when subjected to elevated temperatures during use.

United States Patent 1151 3,655,458 Reichman [451 Apr. 11, 1972 [s41 PROCESS FOR MAKING NICKEL- BASED SUPERALLOYS Ziruary Egzmine -lgvDsavasytgfi Rgtledge sistant amz erar [72] Inventor: Steven H. Reichman, Ann Aoor, Mich. Anomey flamzss Dickey & pierce [73] Assignee: Federal-Mogul Corporation ABSTRACT 22 F1 d: l 10 1970 1 l e y A process for making nickel-based superalloys that possess ex- PP 53,870 cellent high-temperature properties which includes the steps of providing a metal powder having a controlled amount of oxygen and carbon which is confined and densified at an [52] US. Cl utglglggfazggs{351% elevated te-mPer'ature qg a billet can be further 51 Int. Cl. ..c22r 1/10, B22f9/0o, i322f 3/14 firmed! Pmvlde aPPmPnat? [58] Field of Search 148/1 1.5 F 11.5 R 126; Thereafter the i Sublemed 75/203 4 226 heat treatment to effect a growth in the gram size thereof to attain optimum physical properties, whereafter the alloy is carburized to increase the carbon content thereof to a level in [56] References cued excess of about 500 parts per million (ppm) which is per- UNITED STATES PATENTS formed in a manner so as to preferentially promote carhide formatlon at the grain boundanes of the alloy, thereby stabiliz- 3,244,506 4/1966 Reen ..75/O.5 BC i h alloy against f th grain growth when Subjected to 23:5 elevated temperatures during use. 3:556:780 1/1971 110112111. "vs/0513c 10 Claims, 3Drawing Figures BACKGROUND OF THE INVENTION The process comprising the present invention is applicable for forming billets and shaped components possessing excellent physical properties which are comprised of nickel-based superalloys of the type which normally have carbide strengthening and gamma-prime strengthening in their cast and wrought forms. characteristically, these so-called superalloys contain relatively large amounts of second-phase gammaprime and complex carbides in a gamma matrix, which significantly contribute to their high-temperature physical properties. The presence of such complex carbides and second-phase gamma-prime constituents, however, materially increases the difficulty of working and forming cast billets of such alloys into shaped articles. In addition to the difficulty in forming such'alloys, other problems are presented by the'tendency of such alloys to undergo segregation, which is virtually impossible to eliminate due to the magnitude in which it is normally encountered. Such segregation significantly detracts from the high-temperature physical properties of components fabricated from such superalloy materials.

. The foregoing problems associated with cast billets or ingots of superalloy materials have been largely overcome by employing powder metallurgical techniques for forming dense billets of such alloys which are of an extremely fine grain structure. This conveniently is achieved by microcasting or atomizing a melt of the superalloy to a powder state, whereafter it is consolidated under conditions to minimize oxygen entrapment into a blank or billet which approaches 100 percent theoretical density. While such relatively fine grainsized structures exhibit optimum physical properties at temperatures generally below about 1200 F., it is normally such alloys. It has heretofore been discovered that superalloy components prepared by powder metallurgical techniques can be rendered susceptible to grain growth by subjecting them to cold working under controlled conditions, whereafter upon recrystallization and heat treatment, grain growth can be obtained approaching that at which optimum high-temperature physical properties are attained. In some instances, however, it has been observed that certain alloys incorporate secondphase gamma-prime compounds and carbides which have a melting point above the melting point of the gamma phase at the grain boundaries and exhibit other adverse behavior such that grain growth is inhibited even when employing the process as herein-before described. In accordance with the cally and commercially feasible to fabricate superalloy components utilizing powder metallurgical techniques which are characterized as enabling controlled grain growth by heat treatment to attain optimum high-temperature properties consistent with the intended end use of the component.

SUMMARY OF THE INVENTION The benefits and advantages of the present invention are achieved by a process in which a nickel-based superalloy powder is provided which is of a controlled composition so as to contain a maximum of about 200 ppm oxygen and a maximum of about 700 ppm carbon, which is densified at an elevated temperature into a mass or billet approaching 110 .percent theoretical density. The resultant billet can be further deformed, if desired, to attain the appropriate shape and size and thereafter is subjected to heat treatment at an elevated temperature for a period of time sufficient to effect grain growth in the alloy to a size at which optimum high-temperature physical properties consistent with the intended end use are attained. Thereafter, the heat-treated component is carburized to effect a controlled increase in the carbon content thereof to a level of at least about 500 ppm up to about 2000 ppm or greater, effecting a stabilization of the grain structure of the alloy and further enhancing the physical properties thereof. The carburizing treatment is carried out in a manner so as to promote carbide formation preferentially at the grain boundaries of the alloy rather than in the gamma matrix. Following the carburizing treatment, it is usually preferred to subject the alloy to a solution annealing treatment so as to further enhance the homogeneity thereof.

Still other advantages and benefits of the present invention will become apparent upon a reading of the description of the preferred embodiments taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic flow sheet illustrating the sequence of steps involved in accordance with the preferred practice of the process comprising the present invention;

F IG. 2 is a micrograph taken at a magnification of 800 times of a densified nickel-based alloy prior to heat treatment; and

FIG. 3 is a micrograph taken at a magnification of 100 times of the same alloy shown in FIG. 2 afterhaving been subjected to heat treatment at a temperature of 2250 F. for a period of 48 hours to effect a controlled growth in the grain size thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The process comprising the present invention is applicable to a wide variety of well known and commercially available nickel-based alloys. Such superalloys may be further categorized as including those which normally have carbide strengthening and gamma-prime strengthening in their cast and wrought forms. Typically, such nickel-based superalloys, on a weight percent basis, contain from about 5 percent to about 25 percent chromium, about 1 percent to about 10 perdiscovery comprising the present invention, it is now economicent aluminum, about A2 percent to about 10 percent tit mium,

TABLE 1 Nominal compositions of some nickel-base superalloys (percent by weight) Alloy C Cr Al Ti M0 W 00 Cb B Zr Other Ni Nimonic 0. 12 20 0.5 Balance. Nimonic A 0.08 20 1. 5 2. 4 Do. Nimonlc 0.10 20 1.6 2.4 Do. Nimonlc 2.0 3. 0 Do. Nirnonle 0. 5. 0 1. 3 Do. Waspal0y 1.3 3.0 Do. Udimet 700 4. 3 8.5 Do. Rene 41. 1. 5 3. 1 Do. IN-100 (0 5. 5 5. 0 Do. MAR-M200 (cast) 5. 0 2.0 Do. B-1900 (cast. 6.0 1. 0 Do. INCO713 (cast) 6.0 0.75 Do.

2 Maximum.

up to about percent molybdenum, up to about 25 percent tungsten, up to about 25 percent cobalt, up to about 5 percent niobium, up to about 0.07 percent boron, up to about 1.0 percent zirconium, up to about 8 percent tantalum, up to about 1 percent hafnium, up to about 1 percent rhenium and the balance nickel along with conventional impurities in normal amounts. Table 1 contains a listing of specific nickel-based superalloys which are commercially available and are typical of those which can be satisfactorily processed in accordance with the practice of the present invention. L

In accordance with the practice of the present invention, and referring to FIG. lof the drawing, alloys of the aforementioned type are subjected to a processing sequence which normally comprises six steps in the sequence as illustrated. The first step of the process is to provide the nickel-based superalloy of the desired chemical composition in a finely comminuted form, which most conveniently can be achieved by microcasting or atomizing a molten mass of the alloy and thereafter collecting the solidified droplets. The microcasting of the molten alloy can conveniently be achieved as, for example, by utilizing an atomization nozzle and process technique in accordance with that described in US. Pat. No. 3,253,783which is assigned to the same assignee as the present invention, and which patent is incorporated herein by reference. Alternative techniques can be employed for providing the superalloy in a finely particulated state, although gas atomization or microcasting, as it is sometimes referred to, comprises the most convenient and preferred technique.

Regardless of the specific manner in which a comminution of the superalloy material is accomplished, it is a very important aspect of the present invention that the superalloy as initially formulated, as well as during the course of its processing prior to compaction, be controlled such that the oxygen content and the carbon content of the resultant compacted mass be maintained at levels below about 200 ppm and about 700 ppm respectively. In accordance with the preferred practice, the oxygen content is controlled at a level of less than about 100 ppm, while the carbon content should be reduced to a level as low as feasible and preferably to a level less than about 300 ppm. The maintenance of the oxygen and carbon contents below the aforementioned maximum levels is critical in enabling the attainment of appropriate grain size during the heat treatment of the densified billet. Restoration of a desired amount of carbon in the resultant part after the desired grain size has been attained is achieved during the carburizing step in a manner subsequently to be described.

In consideration of the foregoing, when the superalloy powder is produced by the gas atomization of a melt thereof, the atomization of the powder itself, as well as the collection thereof, is achieved under conditions whereby oxygen and other oxygen-containing substances, including water, are substantially completely excluded from contact with the hot powder particles. The particular degree of precaution required to prevent oxidation attack and/or oxygen entrapment during the microcasting operation is, to some extent, dependent upon the particular types and quantity of alloying constituents present in the superalloy. Aluminum and titanium, for example, due to their susceptibility to oxidation attack at high temperature, require increased precaution to exclude oxygen and oxygencontaining substances from contact with the powder particles. The exclusion of such oxygen-containing materials can conveniently be achieved by employing inert atmospheres, such as comprised of argon or helium, for collecting the atomized particles, as well as for effecting an atomization thereof. Commercially available argon and helium gases, which contain minimal amounts of conventional impurities and are substantially moisture-free, have been found satisfactory for producing superalloy powders which contain oxygen in amounts less than 200 ppm and usually substantially less than 100 ppm.

In accordance with conventional gas atomization procedures, the interior of the equipment is initially evacuated and thereafter back-flooded with the substantially dry, nonoxidizing gas, followed thereafter by the gas atomization of the molten superalloy composition. The cooled particles of superalloy powder are generally of a spherical configuration and are of substantially uniform alloy chemistry. The collected powder is subjected to a suitable classification or screening operation in order to separate and collect particles which are of a size generally ranging from about 250 microns (about 60 mesh, US. Standard Sieve Size) down to as small as about 1 micron. Particularly satisfactory results are obtained when the powder particles range in size from about 150 microns (100 mesh) down to about 10 microns and in which the particles are randomly distributed throughout the aforementioned range. Such powders are preferred due to the optimum packing density obtained while in the free-flowing state, which facilitates subsequent densification thereof into a billet approaching 100% theoretical density.

Regardless of the particular technique employed for furnishing the superalloy powder, the resultant powder, having a particle size generally ranging from about 1 micron up to about 60 mesh and containing less than about 200 ppm oxygen and less than about 700 ppm carbon, is next subjected to a densification step in accordance with the sequence illustrated in FIG. 1 of the drawing. The densification of the metallic superalloy powder can be achieved by any one of a variety of techniques well known in the art which includes extrusion, hot upsetting, vacuum die pressing, hot isostatic compaction, explosive compaction, etc. In either case, the specific technique employed is carried out in a manner so as to avoid any appreciable oxidation and carburizing of the powder to assure that the oxygen and carbon contents thereof remain below the aforesaid maximum limits. In such densification processes, the powder is conventionally heated to an elevated temperature in order to facilitate a bond of the powder particles and to further facilitate a compaction and deformation thereof into a billet approaching substantially 100 percent theoretical density. For most nickel-based superalloys, satisfactory for processing in accordance with the present invention, preheat temperatures ranging from about 1900 F. up to about 2500 F. can be satisfactorily employed. The optimum temperature to be used within the aforementioned range is dictated by the temperature sufficient to result in a fully dense product with a structure concomitant with the structure necessary for subsequent forming operations.

In the explosive compaction technique, the powder is subjected to a violent densification by an explosive charge which is usually done without any appreciable preheat of the powder particles prior to consolidation. In the extrusion and hot upsetting compaction techniques, it is conventional to initially confine the powder within a suitable container which is evacuated subsequent to being filled with the powder particles and thereafter sealed to avoid exposure thereof to oxygen and other oxygen-containing substances. Multiple-step compaction processes are also contemplated including the preliminary densification of the free-flowing powder within a die cavity which is subjected to vacuum, producing a pre-form that approaches -90 percent theoretical density. The resultant preform can be subjected to further densification to attain a substantially percent dense billet. Preforms of the foregoing type can also be produced by compacting the free-flowing powder in vacuum and sintering the resultant pre-form at an elevated temperature prior to subsequent further compaction.

Of the foregoing densification processes, the hot extrusion of the powder while disposed within an elongated deformable container constitutes the most convenient and preferred process for forming a densified billet. The container, which is filled with the powder, may comprise any metal which has sufficient ductility to enable the deformation thereof during the extrusion at elevated temperatures without encountering a rupture of the side walls thereof, thereby maintaining the sealed integrity of the powder particles therein. Typical of the various ductile metals that can be satisfactorily employed and which are compatible with the various superalloy powders are the so-called conventional stainless steels, such as AISI Type 304 or an AISI 1010 mild steel. Conventionally, such containers are preliminarily evacuated and thereafter backflooded with a dry, inert gas and the superalloy powder, similarly devoid of any oxygen-containingsubstances, is poured in the container, which thereafter is evacuated and sealed. Optimum packing of the interior of such containers with the free-flowing superalloy powder is usually attained by subjecting the container to sonic or supersonic vibration,

enabling the attainment of usually from 60 percent upwards to 70 percent of theoretical density.

Regardless of the specific compaction process employed, the resultant densified billet of the superalloy powder is characterized as being of a very fine grain size, which is substantially uniform throughout and which, furthermore, contains less than 200 ppm oxygen and less than 700 ppm carbon. The resultant billet, as shown in the flow diagram illustrated in FIG. lof the drawings, normally is subjected to further deformation to provide shaped articles or parts, although the billet itself can be directly subjected to further heat treatment without any further deformation to effect an increase in the grain size thereof. It has been observed that such billets, when of an average grain size of less than about 1 X inch behave in a superplastic manner, enabling deformation thereof into relatively intricate shapes without undergoing any cracking and/or segregation of the structure thereof. It is, accordingly, contemplated within the scope of the present invention that the billet produced after the compaction step can be superplastically deformed after being heated to a temperature above about one-half the absolute melting point of the alloy; can be formed by conventional hot forging techniques; or, when the billet itself is of a usable shape, can be directly subjected to further heat treatment to cause a growth in the grain size thereof in order to provide optimum physical properties consistent with the intended end use of the alloy. A typical microstructure of a superalloy billet is shown in FIG. 2which comprises a micrograph of a Marbles etched sample at a magnification of 800 times. The alloy shown in the micrograph comprises a superalloy containing about 10.76 percent chromium, 6.45 percent aluminum, 4.98 percent titanium, 4.14 percent molybdenum, 17.11 percent cobalt, with the balance essentially all nickel. In addition to the foregoing, the billet is further characterized as containing about 90 ppm oxygen and about 50 ppm carbon. Spherically-shaped powder particles corresponding to the aforementioned alloy composition were microcast and screened, providing powder particles ranging from 10 microns up to 60 microns in size, which thereafter were placed in an elongated cylindrical mild steel container which was evacuated and subsequently sealed. The container and its contents thereafter were preheated to a temperature of about 2100 F. and was extruded at a ratio of about 18:1. The micrograph comprising FIG. 2was taken of a sample of the resultant extruded rod showing a relatively finesized and uniform grain structure.

The next step in accordance with the process comprising the present invention is to subject the billet or shaped part derived from the prior deformation step to a controlled heat treatment in order to effect a growth in the grain size thereof, enhancing the high-temperature physical properties of the alloy. The heat treatment of the billet or shaped part is carried out at a temperature preferably as close to but below the incipient melting point of the gamma matrix which, for most nickel-based superalloys, is within the range of from about 2200 F. up to about 2500 F. This temperature is also generally above the gamma-prime solution or solvus temperature of the various carbide and other complex compounds present at the grain boundary causing them to dissolve and thus further enhancing the grain growth characteristics of the superalloy mass. The duration of the heat treatment step can be varied depending on the grain size desired in the resultant billet. Normally heat-treating periods ranging from about 30 to about 60 hours at temperatures ranging from about 2100 F. to about 2400 F. have been found satisfactory for most of the nickebbased superalloys to which the present invention is applicable, whereby the resultant microstructure of such alloys have a grain size of about one-eighth-inch in diameter. It is feasible, although somewhat impractical, by further continued heat treatment, to effect grain size growth ultimately culminating in a billet or shaped component composed of a single grain crystal.

Typical of the effect which the heat treatment has on the grain size of superalloy billets is that provided by a comparison of the micrographs comprising FIGS. 2and 30f the drawing. As previously indicated, the micrograph of FIG. 2is taken at a magnification of 800 times of a billet as extruded. FIG. 3comprises a micrograph of a Marbles etched sample at a magnification of l00 times of the same billet shown in FIG. 2,but after being subjected to a heat treatment at 2250 F. for a period of about 48 hours.

At the conclusion of the heat treatment step, the large grain-sized superalloy billet or shaped component is preferably subjected to a carburizing treatment in accordance with the arrangement illustrated in FIG. lof the drawing to effect an increase in its carbon content so as to provide for increased carbide strengthening of the microstructure, as well as stabilizing the microstructure against further grain growth when subjected to elevated temperatures during use. The carburizing treatment can be achieved in accordance with any one of the variety of techniques well known in the art and is carried out under conditions which preferentially promote carbide formation at the grain boundaries in comparison to the gamma matrix itself. While any one of the well known carburizing techniques, including pack carburizing, liquid carburizing and gas carburizing, can be satisfactorily employed for this purpose, the most convenient and preferred form for achieving a controlled carburizing of the superalloy billet or component is by the gas carburizing technique utilizing a mixture of natural gas and hydrogen. Alternative well known carrier gases can be employed in the gas carburizing furnace to dilute the hydrocarbon gas to the desired concentration in order to effect a desired degree of carbon addition under the specific time-temperature conditions employed.

It has been found that by controlling the temperature of the superalloy billet or component during the gas carburizing treatment at a level less than about one-half the absolute melting point of the alloy, the carburizing treatment results in a preferential formation of carbides along the grain boundaries of the alloy as opposed to within the gamma matrix itself. This is the case when the alloy is carburized at a temperature below approximately one-half the absolute melting temperature where the diffusion rate is significantly higher for grain boundary regions than matrix regions. In any event, the carburizing treatment is carried out so as to effect an increase in the carbon content of the alloy preferably within the range of from about 500 parts per million up to about 2000 ppm. At levels of less than about 500 ppm, it has been found in some instances that an inadequate quantity of carbides is present at the grain boundaries such that further grain growth in the alloy will occur upon being subjected to elevated temperatures during use. Improved stabilization of the alloy against further grain growth is accomplished at levels above about 500 ppm. When carbon content increases'beyond about 2000 ppm, most superalloy compositions become excessively brittle and, therefore, it is usually preferred to control the carbon content at a level below this magnitude. Generally, the optimum carbon content will be dictated by considerations of the ultimate physical properties desired or required consistent with the specific alloy compositions, the grain size and the stability of the alloy against further grain growth.

As a typical example, the nickel-based alloy produced in accordance with the prior description in connection with FIG. 30f the drawing, having a nominal carbon content of about 50 ppm, was subjected to a carburizing treatment in a gas carburizing furnace maintained at a temperature of 1400 F. for a period of about 7 hours. The carburizing gas mixture comprised 10 percent by volume natural gas and the balance '90 percent by volume hydrogen. As a result of the foregoing carburizing treatment, the nominal carbon content of the alloy was increased to a level above about 170 ppm and the carbides formed were known to be present primarily at the grain boundaries of the alloy. Subjecting this alloy to further carburizing under the foregoing aforementioned conditions results in a further increase in the carbon content thereof above about 500 ppm.

At the conclusion of the carburizing treatment, it is generally preferred, but not necessary, to subject the superalloy billet or shaped component to a high temperature solution annealing treatment to effect an improvement in the homogeneity of the alloy structure. For this purpose, elevated temperatures ranging from about 2000 F. to about 2300 F. and particularly 2100 F. to about 2200 F. have been found satisfactory.

While it will be apparent that the invention herein disclosed is well calculated to fulfill the objects hereinbefore stated, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the spirit thereof.

What is claimed is:

l. A process for making a dense mass of a nickel-based superalloy which comprises the steps of providing a superalloy powder containing less than about 200 ppm oxygen and less than about 700 ppm carbon, densifying said powder into a billet approaching l percent theoretical density, heat treating said billet at an elevated temperature for a period of time sufficient to effect a growth of the grains of the alloy to the desired size, and thereafter carburizing said billet to increase the carbon content thereof in a manner to promote carbide formation preferentially at the grain boundaries of said alloy to an extent to inhibit further grain growth of said alloy when subjected to elevated temperatures.

2. The process as described in claim 1, including the further step of subjecting the carburized said alloy to a solution annealing treatment at an elevated temperature.

3. The process as described in claim 1, including the further step of microcasting the superalloy to provide said powder.

4. The process as described in claim 1, wherein said powder is generally spherical in configuration and is of a particle size ranging from about 1 micron up to about 60 mesh.

5. The process as described in claim 1, wherein the carburizing step is carried out to the extent where the carbon content of said alloy is within a range of from about 500 ppm to about 2000 ppm.

6. The process as described in claim 1, including the further step of deforming said billet to a desired configuration prior to the heat treatment thereof.

7. The process as described in claim 1, in which said powder contains less than about ppm oxygen and less than about 300 ppm carbon.

8. The process as described in claim 1, wherein said billet derived from said densifying step has a grain size of less than about 1 X l0" inch and including the further step of superplastically deforming said billet to a desired configuration prior to the heat treatment thereof.

9. The process as described in claim 1, wherein the densification of said powder into said billet is achieved by confining said powder in a sealed deformable container and compacting said powder while in said container which is preheated to a temperature ranging from about l900 F. to about 2500 F.

10. The process as described in claim 1, in which the carburization of said billet is accomplished by a gas carburizing technique carried out at a temperature of less than about onehalf the absolute melting point of said alloy. 

2. The process as described in claim 1, including the further step of subjecting the carburized said alloy to a solution annealing treatment at an elevated temperature.
 3. The process as described in claim 1, including the further step of microcasting the superalloy to provide said powder.
 4. The process as Described in claim 1, wherein said powder is generally spherical in configuration and is of a particle size ranging from about 1 micron up to about 60 mesh.
 5. The process as described in claim 1, wherein the carburizing step is carried out to the extent where the carbon content of said alloy is within a range of from about 500 ppm to about 2000 ppm.
 6. The process as described in claim 1, including the further step of deforming said billet to a desired configuration prior to the heat treatment thereof.
 7. The process as described in claim 1, in which said powder contains less than about 100 ppm oxygen and less than about 300 ppm carbon.
 8. The process as described in claim 1, wherein said billet derived from said densifying step has a grain size of less than about 1 X 10 4 inch and including the further step of superplastically deforming said billet to a desired configuration prior to the heat treatment thereof.
 9. The process as described in claim 1, wherein the densification of said powder into said billet is achieved by confining said powder in a sealed deformable container and compacting said powder while in said container which is preheated to a temperature ranging from about 1900* F. to about 2500* F.
 10. The process as described in claim 1, in which the carburization of said billet is accomplished by a gas carburizing technique carried out at a temperature of less than about one-half the absolute melting point of said alloy. 